Bone repair often involves the use of orthopedic implants to replace missing bone or support bone during the healing process. It is typically desirable to coat such orthopedic implants with osteoconductive materials to encourage bone growth or biological fixation.
Hydroxyapatite (HA) is a naturally occurring mineral found in bones and teeth. Studies have shown that HA is osteoconductive, and orthopedic implants have been coated with HA for this reason. Octacalcium Phosphate has also been reported to be osteoconductive. Various processes for coating implants with CaP and HA are known. One process used for coating implants is plasma spray. In this process, HA powder is fed into a high temperature torch with a carrier gas. The HA powder is partially melted and then impacts the substrate at high velocity whereupon it is rapidly quenched back to room temperature. This process produces a mixture of HA, other calcium phosphate phases, and amorphous calcium phosphate. These phases have wide differences in solubility in vivo. As a result, plasma sprayed hydroxyapatite (PSHA) films do not uniformly dissolve or degrade in vivo. This non-homogenous degradation can generate particulates in the vicinity of the implant which can result in an inflammatory cascade leading to osteolysis. The particles may also find their way into joint articular surfaces, resulting in increased wear. Finally, the process is not well suited for coating porous structures of cementless implants because it is a “line of sight” process. PSHA processing or post processing methods can be applied that result in highly crystalline coatings with long resorption times in-vivo. This attribute gives rise to concerns over long term delamination of these relatively thick stable coatings.
Other methods to produce HA coatings for biological fixation include physical methods such as sputtering, evaporation, and chemical vapor deposition. These physical methods do not reproduce the nano-crystallinity and high surface area of biological apatites, and the resulting coatings may not uniformly dissolve and may release particulates.
Solution (or suspension) methods for producing HA coatings have also been attempted. For example, Zitelli, Joseph P. and Higham, Paul (2000), A Novel Method For Solution Deposition of Hydroxyapatite Onto Three Dimensionally Porous Metallic Surfaces: Peri-Apatite HA describes a process that involves producing a slurry of finely divided HA particles into which implants are placed and coated by accretion of the slurry particles. High surface area, microcrystalline coatings are produced, but their adhesion to the substrate is poor.
Electrochemically assisted solution deposition has also been developed. In this process, a voltage exceeding that necessary to hydrolyze water is applied to an implant while the implant is suspended in an aqueous solution. This process results in deposition of calcium phosphate material on the implant. Typically, the deposited film is a mixture of calcium phosphate (CaP) phases and requires post processing to convert the films to phase pure HA. Poor adhesion is also a concern with these films. Finally, control of electrochemical currents on porous implants with irregular particles is challenging, making this process difficult to scale.
Biomimetic processes have also been developed. These processes employ solutions mimicking body fluid concentrations and are typically performed near body temperature. These processes can yield bone-like apatite but require days or weeks to produce films a few microns thick. Attempts to increase rates associated with such methods have led to complications in reproducibly controlling pH, deposition rate, and accretion rate compared to crystalline growth on the surface of the implant. Films formed at higher rates have been found to contain amorphous material. Uncontrolled deposition rate also makes it difficult to achieve target coating weights or thicknesses.
As described above, hydroxyapatite coatings may be applied to orthopedic implants to enhance osteoconductivity using methods that are either rapid but lead to coatings having certain undesirable or unpredictable properties or lead to more desirable products but can take days to form. What is needed is a conformal calcium phosphate coating that can be rapidly formed and has a microstructure that lends itself to uniform degradation over a period of several weeks without generating particulates.